A few days ago, I encountered a case, a 11.2.0.4 three-node Oracle RAC database on RHEL 6.6 , when trying to login to the database instance using sqlplus “/ as sysdba” on the third node, It’s very slow, and vmstat show that there is a very large swap in and out, but there is still a lot of memory free space

If kernel can not find memory to allocate when it’s needed, it puts in-use user data pages on the swap-out queue, to be swapped out. If the Virtual Memory (VM) cannot allocate memory and canot swap out in-use memory, the Out-of-memory killer may begin killing current userspace processes.